Increasingly, vehicles are being powered by an electric motor, either exclusively, or in conjunction with another power source, such as an internal combustion engine. In such vehicles, a high-voltage electrical power source—e.g., a high-voltage battery—is used to power the electric motor and other high voltage loads within the vehicle. In addition to the high voltage battery, a hybrid electric or electric vehicle may also have a low-voltage battery, which may be used to power vehicle lighting, engine cooling fans, heated seats, and/or other low-voltage loads. It may be necessary to provide power to some of the low-voltage loads when the vehicle is not operating—i.e., when the vehicle is in a “key-off” state.
In order to maintain the low-voltage electrical loads during the key-off state, it is necessary to provide a low-voltage power source having an appropriate capacity. The greater the level of the low-voltage loads, and the longer it is desired to maintain those loads, the greater the required capacity of the low-voltage power source. For example, it may be desirable to supply power to the key-off low-voltage loads for several weeks while the vehicle is idle and not being driven. In such a case, the capacity of the low-voltage power source must be relatively high.
Because there is generally a relationship between the capacity of a battery and its physical dimensions, packaging a low-voltage battery of an appropriate capacity can be problematic. In particular, the under-hood compartments of vehicles today, which are already closely packed, may not have the space necessary to accommodate a low-voltage battery. Therefore, it may be necessary to choose alternative locations for the battery. This can undesirably increase the complexity of the wiring within the vehicle, as well as vehicle cost. Therefore, it would be desirable to be able to supply power to low-voltage key-off vehicle loads for a desired amount of time, while keeping the size of the low-voltage power source small enough to be located under the hood or within close proximity thereof.